All of the above hook into the mysql server by setting the HOST and PORT settings in the django settings.

Each slave celery box uses an environment variable to take care of any individual settings it might need, but in general each of them uses django-celery's BROKER_HOST and BROKER_PORT options to connect to the rabbitmq server.

using fabric makes deploying code to all of them fairly simple

Believe it or not, rabbitmq effortlessly figures out who's got a free worker between all of your boxes and just does it.